Prefix override

- add setup for prefix override through initializer
Update README.md

add gem version icon
Tests

- batch-rails2 is a module
- prefix settings can be set via initializer
Merge branch 'master' of github.com:mortik/batch-rails2
Update README.md
test log to gitignore and optimize settings test
Merge branch 'master' of github.com:mortik/batch-rails2
loosen dependency versions
v0.0.4